1. Field of the Invention
The present invention relates to an ultrasonic flow velocity measurement method and apparatus thereof. More particularly, the present invention relates to an ultrasonic flow velocity measurement method in which ultrasonic wave transducers are installed on the outer surface of a pipe and the flow velocity of a fluid at a high temperature and pressure or at a low temperature inside the pipe is measured, and to an apparatus thereof.
2. Description of the Related Art
An example of the prior art shown in FIGS. 9 and 10 is disclosed in U.S. Pat. No. 4,930,358.
In these figures, reference numeral 51 denotes one of the ultrasonic wave transducers mounted on the upstream side of a pipe 3, and reference numeral 52 denotes the other ultrasonic wave transducer mounted on the downstream side of the pipe 3. The ultrasonic wave transducer 51 comprises a wedge member 51A for making ultrasonic waves enter the pipe 3 obliquely and a vibrator 51B, as shown in FIG. 10. The wedge member 51A is formed of an acrylic resin and the cross section thereof is shaped as a trapezoid. The ultrasonic vibrator 51B is fixedly mounted on one of the inclined surfaces 51a thereof. The other inclined surface 51C forms an ultrasonic reflection surface intersecting at right angles with a propagation path when ultrasonic waves originated from the vibrator 51B are reflected by a surface 51b serving as an ultrasonic incident surface and the ultrasonic waves are propagated within the wedge member 51A. Hence, internal reflection waves propagated inside the wedge member 51A return to the vibrator 51B.
If this propagation time is denoted as t.sub.p, a sound velocity C.sub.1 within a wedge inside the ultrasonic wave transducers 51 and 52 can be determined by the following equation: EQU C.sub.1 =2(l.sub.1 +l'.sub.1)/(t.sub.p -.tau..sub.e) 1
where l.sub.1 and l'.sub.1 are the length of the paths l.sub.1 and l'.sub.1 shown in FIG. 10, respectively, and .tau..sub.e is an electrical delay time inside a cable or the like. The length b of the incident surface 51b which serves as an opening surface when ultrasonic waves inside a pipe are emanated, is set at almost 18 wavelengths or even greater with respect to the central frequency used.
Since a directional angle becomes very small when the opening of the vibrator 51B is somewhat long and the length b of the incident surface 51b is almost 18 wavelengths or even greater as shown in FIG. 9, an ultrasonic beam, which is propagated in turn in the wedge section inside the ultrasonic wave transducer, in the pipe section and in the fluid section inside the pipe, is regarded as a substantially parallel beam.
Ultrasonic waves outputted from the vibrator 51B of the ultrasonic wave transducer 51 toward the downstream side pass through a propagation path consisting of parallel beams shown by a slanted line in FIG. 9 and reach the ultrasonic vibrator 52B of the other ultrasonic wave transducer 52. The propagation time in this case is denoted as t.sub.d.
Ultrasonic waves outputted from the vibrator 52B of the ultrasonic wave transducer 52 toward the upstream side reach an ultrasonic vibrator 51B of the other ultrasonic wave transducer 51. The propagation time in this case is denoted as t.sub.u.
In such a case, a flow velocity V inside the pipe 3 can be determined by the following equation: ##EQU1## where Lx is the distance between the intersecting points of the inclined surfaces 51a and 52a where vibrators contact the incident surfaces 51b and 52b, i.e., between 1R and 2R; .theta..sub.1 is an incident angle of the ultrasonic waves within the wedge; d is the plate thickness of the pipe; D is the internal diameter of the pipe; and N is the number of passages of the ultrasonic waves inside the fluid. In the case of FIG. 9, N=2. C.sub.2 is a predetermined sound velocity inside the pipe wall.
As a result, even if the sound velocity C.sub.3 of a fluid is unknown, the flow velocity of the fluid inside the pipe 3 can be measured relatively easily on the basis of equations 2 and 3. At the same time, since the internal diameter of the pipe 3 is known, the quantity of flow of the fluid inside the pipe 3 can be determined quite easily.
Furthermore, incident points inside an ultrasonic wave transducer need not be strictly specified. Since it is necessary to know only the mounting distance Lx, setting the ultrasonic wave transducer is quite easy.
In addition, since ultrasonic waves emanated from the two ultrasonic wave transducers 51 and 52 have quite small directional angles, the transducers are immune to influences of the resonance mode (plate waves) of the pipe 3 which often becomes a problem in measuring.
However, such a conventional ultrasonic flow velocity measuring method and apparatus thereof cannot be used to measure the flow velocity of a fluid in a high temperature and pressure state (or in a low temperature state), if the construction thereof is not changed, since the vibrators 51B and 52B of the ultrasonic wave transducers 51 and 52 have a maximum operating temperature limit.
In addition, since a velocity gradient is caused in the sound velocity C.sub.1 within the wedge because of the temperature gradient caused within this wedge, the sound velocity C.sub.1 determined from t.sub.p by using equation 1 indicates an average sound velocity within the wedge. The sound velocity C.sub.1 has a problem (drawback) in that a large error occurs when the flow velocity of a fluid at a high temperature and pressure is measured, since the sound velocity C.sub.1 is obviously different from a sound velocity in which Snell's law is applied because a propagation path for ultrasonic waves is indentified.